Ametropia, an undesirable refractive condition of the eye, has three main subdivisions: myopia, hyperopia, and astigmatism. In myopia, by far the most common type of ametropia, the parallel light rays 20 which enter the eye as shown in FIG. 1 come to a focus F1 in front of the retina 24 as shown in FIG. 2. In hyperopia, the rays of light 20 come to a focus F2 behind the retina 24 as shown in FIG. 3. When the rays of light converge to not one, but several foci, it is referred to as astigmatism, in which condition the various foci may all lie before the retina; all lie behind the retina; or partly before and partly behind the retina.
Ametropia is usually corrected by glasses or contact lenses. However, these refractive disorders may also be corrected by surgery. Refractive eye surgery is defined as that surgery on the eye which acts to change the lightbending qualities of the eye. More common current refractive procedures include radial keratotomy, as described in U.S. Pat. Nos. 4,815,463 and 4,688,570 and also laser ablation of corneal stroma, described in U.S. Pat. No. 4,941,093. Various other surgical methods for the correction of refractive disorders have been tried including thermokeratoplasty for the treatment of hyperopia, epikeratoplasty to correct severe hyperopia, and keratomileusis which can steepen or flatten the central cornea. Keratomileusis was introduced by Barraquer of Colombia in 1961 and essentially involves grinding a corneal button into an appropriate shape to correct the refractive error and replacing the reshaped corneal button. Some of the more common keratorefractive procedures are discussed below, none of which have currently shown itself to have all the characteristics of an ideal keratorefractive procedure.
In radial keratotomy (RK) multiple peripheral radially directed incisions are made into the cornea at 90-95% depth in an attempt to flatten the central cornea and thus correct myopia. The problem of unpredictability of result was tackled by multiple extensive retrospective analyses of the patients in whom surgery had already been performed. These studies revealed certain factors that seemed to control the outcome of the surgery, such as the size of the optical zone, the initial keratometric readings, corneal diameter, corneal rigidity, number of incisions, incisional depth, intra-ocular pressure, thickness of the cornea, and degree of astigmatism. Age and sex are also factors that are taken into consideration in most of the nomograms which have been devised to predict what effect to expect for a certain surgery. At one point, many experts in the field considered it nearly impossible to fully and accurately correct patients in one surgery and felt that RK should be considered a two-stage surgery, with the initial surgery to achieve the "ball-park" correction, followed by an enhancement procedure to fine-tune the original correction. It was felt that because of individual variability which may lead to an under or overcorrection in the individual different from that predicted by the nomogram, attempting to fully correct the refractive error in one surgery could lead to over-correction in a not insignificant percent of the surgeries, resulting in hyperopia which is much more difficult to correct. Unfortunately, the second-stage surgery is even less predictable than the initial procedure. No one has yet devised a formula to take into account the profound changes which occur in the cornea after the initial RK, especially when weeks or months have passed. It is obvious that RK does not qualify as a simple, safe, predictable procedure to adjust the refractive outcome after the initial RK has been performed. Most ideas to contend with the corneal shape after this event have been purely empiric. Thus an easy method to fine-tune a refractive correction that is minimally invasive and easily performed, would require serious consideration.
Laser stromal ablation procedures, such as photorefractive keratectomy (PRK) for correction of refractive disorders are currently popular and have had reasonable success. These procedures are not, however, spared from the problem of unpredictability. Essentially, in the treatment of myopia, laser energy is imparted to the central cornea thereby causing excision of more tissue centrally and a resultant flattening of the cornea. Unfortunately, the final refractive effect is determined not only by the amount of ablation but also by the healing response to the keratectomy. The cornea actively lays down new collagen and the epithelium undergoes a hyperplastic response, among other responses, in an attempt to repair the damage to its surface. This causes regression, or a shift backwards towards myopia, which can gradually occur over a period of months to years. An undesired effect of new collagen deposition is stromal scar formation which manifests as stromal haze and possible decrease in contrast sensitivity by the patient. Predictability with PRK is an issue, as with RK. Most published results of outcome after PRK treatment for myopia show 80-94% of eyes obtaining uncorrected visual acuity of 20/40 or better while the percentage of patients achieving 20/20 is significantly less. Although visual recovery is slow in RK, it is quicker than after PRK. A second laser ablation procedure is usually undertaken with caution since it may cause a greater healing response with even more regression than the initial procedure. Again, as in RK, the laser ablation procedure is not completely predictable, partly because one cannot predict an individual's wound healing response.
For years it has been thought that refractive surgery with intracorneal implants could be used in the correction of ametropia. Early techniques included lamellar removal or addition of natural corneal stromal tissue, as in keratomileusis and keratophakia. These required the use of a microkeratome to remove a portion of the cornea followed by lathing of either the patient's (keratomileusis) or donor's (keratophakia) removed cornea. The equipment is complex, the surgical techniques difficult, and most disappointingly, the results quite variable. The current trend in keratorefractive surgery has been toward techniques that are less traumatic to the cornea, that minimally stimulate the wound healing response, and behave in a more predictable fashion. The use of alloplastic intracorneal lenses to correct the refractive state of the eye, first proposed in 1949 by Jose Barraquer, have been plagued with problems of biocompatibility, permeability to nutrients and oxygen, corneal and lens hydration status, etc. Other problems with these lenses included surgical manipulation of the central visual axis with the concomitant possibility of interface scarring.
More recent efforts toward the correction of refractive errors have focused on minimizing the effects of the wound healing response by avoiding the central cornea. There have been multiple attempts to alter the central corneal curvature by surgically manipulating the peripheral cornea. These techniques are discussed because of their specific relevance to this invention. The general concept of making fixed changes in the corneal curvature was adapted by A. E. Reynolds in a unique way (U.S. Pat. No. 4,452,235). He describes and claims a keratorefractive technique involving a method and apparatus for changing the shape of the optical zone of the cornea to correct refractive error. His method comprises inserting one end of a split ring shaped dissecting member into the stroma of the cornea, moving the member in an arcuate path around the cornea, releasably attaching one end of a split ring shaped adjusting member to one end of the dissecting member, reversibly moving the dissecting member about the path, and thereby pulling the adjusting member about the circular path, made by the dissecting member, withdrawing the dissecting member, adjusting the ends of the split ring shaped adjusting member relative to one another to thereby adjust the ring diameter to change the diameter and shape of the cornea and fixedly attaching the ring's ends by gluing to maintain the desired topographical shape of the cornea.
A major advantage of this ring was that a very minimal wound healing effect was expected. A marked corneal wound healing response would decrease the long-term stability of any surgical refractive procedure. However, there are two distinct problem areas affecting the refractive outcome of surgical procedures treating ametropia.
1. The first area involves the ability to predetermine the shape and size of a ring that will lead to a certain refractive outcome. In RK or PRK, retrospective studies have been performed that led to the development of nomograms which predict that a certain depth cut or a certain ablation amount will result in a predictable amount of correction. In the case of the ring, eventually nomograms will be developed that can be used to predict a given refractive correction for a given thickness or size of the ring. However, these nomograms can never fully account for individual variability in the response to a given keratorefractive procedure.
2. The refractive outcome also depends on the stability of the refractive correction achieved after surgery. To reiterate, the advantage of the ring would be the stability of the refractive outcome achieved because of a presumed minimal wound healing response. This decreases the variability of the long-term refractive outcome but still does not address the problems posed in the first problem area, --the inherent individual variability, in that while the outcome may be stable, it may very well be an inadequate refractive outcome that is stable.
Another unaddressed issue is that even with the ring, surgeons will aim for a slight undercorrection of myopia because, in general, patients are more unhappy with an overcorrection that results in hyperopia. Again, the refractive outcome may be more stable than in RK or PRK but it may be an insufficient refractive result that is stable.
Simon in U.S. Pat. No. 5,090,955 describes a surgical technique that allows for modification of the corneal curvature by interlamellar injection of a synthetic gel at the corneal periphery while sparing the optical zone. He does discuss an intra-operative removal of gel to adjust the final curvature of the central corneal region, but it has yet to be shown that the gel injection keratoplasty results in a stable refractive outcome. Also, a post-operative adjustment is not disclosed, but it would be difficult at best with such a technique.
Siepser (U.S. Pat. No. 4,976,719) describes another ring-type device to either flatten or steepen the curvature of the cornea by using a retainer ring composed of a single surgical wire creating a ring of forces which are selectively adjustable to thereby permit selective change of the curvature of the cornea, --the adjustable means comprising a turnbuckle attached to the wire.
Silvestrini et al. in U.S. Pat. No. 5,466,260 describes an adjustable intrastromal corneal ring (ICR) but states that the essence of the invention lies in the ability of the ring to be adjusted in thickness so that it is not necessary to stock a plurality of different rings of different sizes. It becomes obvious that the rings are not designed to be adjusted within the cornea at a later post-operative date. Silvestrini et al further states that "it may not be further adjustable after insertion." The idea of an inflatable ring implant was raised by Silvestrini as a way to decrease the number of different size rings that would need to be stocked, but he does not mention the idea of an inflatable ring that could be adjusted following surgery.
There are several mechanisms by which peripheral manipulation of the cornea affects anterior corneal curvature. The cornea, like most soft tissues, is nonlinear, viscoelastic, nonhomogeneous, and can exhibit large strains under physiologic conditions. The whole eye is geometrically extremely complex and the biomechanics technique capable of systematically modeling this reality is the finite element method which assumes small strains (a measure of deformity), homogeneity, and linear elastic behavior. Two simple mechanisms will be briefly described.
A simple example is helpful in understanding the first mechanism. Assume a loose rope R between two fixed points P1 and P2 as in FIG. 4a, which forms a curve, the lowest point P being in the middle. Referring to FIG. 4b, a weight W placed on the rope between the middle point P and one fixed point will cause the central portion of the rope to straighten. The cornea C demonstrated in FIG. 5(a) and FIG. 5(b) behaves similarly, the two fixed points, P1 and P2, analogous to the limbus of the eye and the weight W similar to the intrastromal ring 30 which, when inserted in the cornea in surrounding relation to the cornea's central optical zone, causes the corneal collagen fibers to deviate upwards at (21) above the ring, and downwards at (22) below the ring. In essence, this deviation of the cornea around the peripheral ring causes other areas of the cornea to lose "slack", or relatively straighten as shown at (23).
Mechanical expansion of the ring diameter as shown by expansion of the ring 30 in FIG. 6(b) as compared to FIG. 6(a) also flattens the central corneal curvature whereas constriction of the ring 30 steepens the central corneal curvature, analogous to the two fixed points in the example, FIG. 4(a) and FIG. 4(b), being moved together and causing the rope in the middle to sag more. This is permitted to occur, in part, because the boundary nodes at the limbus are not completely fixed. In summary, there is a microdeviation caused by the bulk of the ring 30 itself within the peripheral tissue, slightly flattening the central curvature of the cornea, and a constricting or expanding ring altering the fixed points and thus altering corneal curvature. A constricting or expanding ring is likely to cause a less stable refractive outcome because the inward or outward forces of the ring against the corneal stroma may gradually cause further lamellar dissection and dissipation of the forces. A more consistent outcome is likely to be achieved with varying the thickness of the ring itself.
The second mechanism is aptly described by J. Barraquer in the following quote. Since 1964, "It has been demonstrated that to correct myopia, thickness must be subtracted from the center of the cornea or increased in its periphery, and that to correct hyperopia, thickness must be added to the center of the cornea or subtracted from its periphery." Procedures involving subtraction were called "keratomileusis" and those involving addition received the name of "keratophakia". Intrastromal corneal rings add bulk to the periphery and increasing the thickness of the ring results in a more pronounced effect on flattening of the anterior corneal curvature.
The ideal keratorefractive procedure allows all the advantages of eyeglasses or contact lenses, namely, being able to correct a wide range of refractive errors, accuracy or predictability, allowing reversibility in the event that the refractive state of the eye changes and it becomes necessary to adjust the correction again, yielding minimal complications, and associated with technical simplicity, low cost, and being aesthetically acceptable to the patient.
Once again, an easy procedure to post-operatively fine-tune the refractive correction and corneal curvature which is often influenced by changes in corneal hydration status, wound healing responses, and other unknown factors, is not available. In this disclosure of the present invention, an easy method to adjust the refractive outcome after the corneal curvature has stabilized, a method that is minimally invasive, a method causing minimal stimulation of the wound healing processes, allowing repetitive adjustments as deemed necessary, and being almost completely reversible is described. It may make moot the pervasive issue of unpredictability and make obsolete the application of procedures which rely heavily upon nomograms to predict refractive outcome and are thus unable to adequately account for an individual's variable response to the procedure.